Photoreceptors are a specialized subset of retinal neurons, that are responsible for vision. Photoreceptors consist of rods and cones which are the photosensitive cells of the retina. Each rod and cone elaborates a specialized cilium, referred to as an outer segment, that houses the phototransduction machinery. The rods contain a specific light-absorbing visual pigment, rhodopsin. There are three classes of cones in humans, characterized by the expression of distinct visual pigments: the blue cone, green cone and red cone pigments. Each type of visual pigment protein is tuned to absorb light maximally at different wavelengths. The rod rhodopsin mediates scotopic vision (in dim light), whereas the cone pigments are responsible for photopic vision (in bright light). The red, blue and green pigments also form the basis of color vision in humans. The visual pigments in rods and cones respond to light and generate an action potential in the output cells, the rod bipolar neurons, which is then relayed by the retinal ganglion neurons to produce a visual stimulus in the visual cortex.
In humans, a number of diseases of the retina involve the progressive degeneration and eventual death of photoreceptors, leading inexorably to blindness. Degeneration of photoreceptors, such as by inherited retinal dystrophies (e.g., retinitis pigmentosa), age-related macular degeneration and other maculopathies, or retinal detachment, are all characterized by the progressive atrophy and loss of function of photoreceptor outer segments. In addition, death of photoreceptors or loss of photoreceptor function results in partial deafferentation of second order retinal neurons (rod bipolar cells and horizontal cells) in patients with retinal dystrophies, thereby decreasing the overall efficiency of the propagation of the electrical signal generated by photoreceptors. Secondary glial and pigment epithelium changes secondary to photoreceptors degeneration result in vascular changes leading to ischemia and gliosis. Trophic factors that are capable of rescuing photoreceptors from cell death and/or restoring the function of dysfunctional (atrophic or dystrophic) photoreceptors may represent useful therapies for the treatment of such conditions.
The progression of these conditions points to a sequential loss of the two classes of photoreceptors: initially rods are lost as a direct result of a genetic or environmental or unknown lesion, resulting in night blindness and a reduction in visual field followed inevitably by loss of cones leading to total blindness. Thus, cones die indirectly since they do not express the primary lesion.
Not all of the genes associated with retinal dystrophy have yet been identified. Identification of such genes would make possible to both diagnose the disease and identify effective therapies.